A conventional vehicle air-conditioning apparatus is described, for example, in JP-A-2001-213152. This vehicle air-conditioning apparatus has a humidity sensor for detecting relative humidity in a passenger compartment. The vehicle air-conditioning apparatus calculates humidity in the vicinity of the inner surface of a vehicle window through the use of the value detected by the humidity sensor. Moreover, on the basis of the calculated humidity, it is determined whether the vehicle window becomes fogged or not. When determined to become fogged, its defogging system is activated so as to defog the vehicle window, e.g., its evaporator is driven more actively so as to heighten its dehumidifying performance.
However, in this vehicle air-conditioning apparatus, the humidity sensor is likely to incorrectly detect the relative humidity in the passenger compartment. When the humidity sensor detects the value of the relative humidity more highly than its true value, the vehicle window is erroneously determined to be fogged. Accordingly, the defogging system is unnecessarily and wastefully driven. Otherwise, when the humidity sensor detects the value of the relative humidity lower than its true value, the vehicle window is erroneously determined not to be fogged. Accordingly, the vehicle window disadvantageously becomes fogged.